How do you get your coupon code?

From the Spoordeelwinkel

Third parties

Supermarkets, department stores and other enterprises often sell cheap day tickets for the train. These sometimes come with a coupon code. You can use this coupon code to purchase a discounted NS ticket.

It is also possible that NS is having a promotion and you receive a coupon code. You can then get a discount by entering the coupon code when you buy an e-ticket online.